The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit having layers containing an organic polymer called "kerogen", which upon heating decomposes to produce hydrocarbon liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen bearing shale and processing the shale above ground, or processing the oil shale in situ.
The recovery of liquid and gaseous products by processing oil shale in situ has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972 to Donald E. Garrett, and assigned to the assignee of this application. This patent describes in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale by mining out a portion of the subterranean formation and then fragmenting a portion of the remaining formation to form a stationary, fragmented permeable mass of formation particles containing oil shale, referred to as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products.
Rather than discarding such mined oil shale, it is desirable to retort the mined oil shale above ground to recover liquid and gaseous hydrocarbon materials. Exemplary of processes for retorting oil shale above ground is the process described in U.S. Pat. No. 2,908,617 issued to Murphree. In this process, raw oil shale is heated to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products and a residue of solid carbonaceous material. In the Murphree process the heat for retorting is obtained by oxidizing residual carbonaceous material in the retorted oil shale in a combustion zone with concomitant generation of a flue gas. The heat content of the flue gas is not utilized. Hot shale formed in the combustion zone is cycled to the retorting zone to provide the heat necessary for retorting.
Processes such as that of Murphree are thermally inefficient because the heat content of the flue gas is lost from the process. This can reduce the yield of hydrocarbons obtained from the oil shale because instead of using the thermal energy of the flue gas for retorting oil shale, thermal energy for retorting oil shale is obtained by oxidation of hydrocarbon products.
Therefore, there is a need for a high efficiency method for recovery of hydrocarbon values from oil shale.